The present invention relates to a technique for transmitting or delivering a laser beam using an optical fiber and particularly relates to a laser transmission system particularly of an optical fiber transmission type laser system including a pulse laser oscillator, a beam guide device and an optical fiber capable of transmitting a pulse laser beam having a high peak output in a good state while preventing the optical fiber from being damaged.
In the recent years, in order to ensure preventive maintenance of structures within a nuclear reactor and to intensify the strength of various types of members required to provide high durability, the development of laser peening technique has been progressed and its application is partly considered. In the laser peening apparatus, a pulse laser beam having a peak output as high as, for example, 10 MW or more is employed. The development of a technique for removing the surface layer of a metal material, analyzing elements in fine amount (i.e. microanalyzing elements) or the like using such a pulse laser beam of high peak output has been also performed.
In an apparatus employing a pulse laser beam having a high peak output such as a laser peening apparatus, a spatial (space) transmission using a reflection mirror is normally used as pulse laser beam transmission means. However, if a transmission path is complex and long, or the degree of freedom of a transmission path is required, it is necessary to use many reflection mirrors and accurately manage or control the positions and angles of the mirrors, which results in that the apparatus disadvantageously becomes complicated. In a laser peening apparatus for preventive maintenance of structures in a nuclear reactor, in particular, it is necessary to remotely control the above-stated reflection mirrors to reduce the possibility of exposing operators to radiation. As a result, the apparatus becomes more complicated. Further, in spatial transmission using the reflection mirrors, it is extremely difficult to transmit a pulse laser beam through a long narrow portion such as the interior of a tubing having a diameter of less than 1 cm.
To solve the above problem, demand for transmission using an optical fiber capable of easily realizing beam transmission having high degree of freedom rises. However, when a pulse laser beam of high peak output is introduced inside the optical fiber, the peak output is extremely high and the optical fiber itself may be possibly damaged. As means for avoiding such defect, it has been recently considered to use a beam guide device using the optical fiber by means of image formation or use a taper fiber.
First, an example of a beam guide device by means of image formation will be described. FIG. 22 shows the structure of the device. This device includes an aperture 1 (diameter d), a coupling lens 2 having a magnification m and an optical fiber 3 held by an optical fiber holder 3a sequentially arranged on the optical path of a pulse laser beam L. An aperture surface A and an optical fiber end face B are arranged to be conjugated with each other (or to have image-formation relationship). The position of the aperture 1 is set such that an extreme peak is not included in the energy density distribution of a cut pulse laser beam L.
In the device constituted as stated above, a pulse laser beam L is cut by the aperture 1 (beam diameter d) and the cut image is reduced by the image-formation lens 2 (beam diameter md) and projected onto the optical fiber end face B (core diameter a greater than md). In such structure, the pulse laser beam L does not include the extreme peak of the energy density at the aperture cutting position. According to this matter, the extreme peak of the energy density is not included in the optical fiber end face B and it is, therefore, possible to avoid the damaging of the optical fiber 3 on the optical fiber end face B.
Next, an example of a taper fiber will be described. FIG. 23 shows the structure of the taper fiber. The taper fiber 4 to be used is an optical fiber having a tapered cross section on the beam incidence side end portion, and the core diameter a0 of the fiber 4 on the end face is larger than the core diameter a of a fiber main portion (a0 greater than a).
A pulse laser beam L is first condensed (converged) by a condensing lens 5 and then applied onto the end face of the taper fiber 4, with a beam diameter d larger than the core diameter a of the optical fiber at the central portion and smaller than the core diameter a0 on the end face (a0 greater than d greater than a). Accordingly, the energy density of the pulse laser beam L on the optical fiber end face can be reduced, and therefore, the optical fiber on the end face can be prevented from being damaged.
Meanwhile, in the beam guide device by means of image formation shown in FIG. 22, it is possible to avoid the damaging of the optical fiber by removing the local peak of the energy density on the optical fiber end face. Within the optical fiber, however, a laser beam having a high directivity is reflected on the interface between a depressed core and a clad, so that the laser beam is converged finely to thereby damage the optical fiber.
Furthermore, even with the taper fiber shown in FIG. 23, although it is possible to avoid the damaging of the optical fiber by reducing the energy density of a laser beam on the optical fiber end face, the laser beam is converged within the optical fiber by the same function as that mentioned above, and the optical fiber is thereby damaged.
As mentioned above, it has been recently desired to transmit a pulse laser beam of a high peak output by using an optical fiber. However, any means for realizing such desire has not yet been provided.
As described above, although conventional techniques can avoid the damaging of an optical fiber on the end face thereof, the problem with these techniques is that they cannot prevent the damaging of the optical fiber resulting from the convergence of the laser beam in portions other than the optical fiber end face.
It is an object of the present invention to substantially eliminate defects or drawbacks encountered in the prior art mentioned above and to provide a laser transmission system for transmitting laser beam by means of an optical fiber capable of transmitting or delivering a pulse laser beam of high peak output without damaging the optical fiber.
This and other objects can be achieved, according to the present invention, by providing a laser transmission system using an optical fiber, comprising:
a pulse laser oscillator unit;
a beam guide unit having an optical condensing unit for condensing a pulse laser beam radiated from the pulse laser oscillator unit;
an optical fiber unit for transmitting the pulse laser beam condensed by the beam guide unit; and
means for reducing a coherence of the pulse laser beam provided for at least one of said pulse laser oscillator unit, said beam guide unit and said optical fiber unit.
In this aspect, the coherence reducing means is a means for making substantially uniform distribution of laser beams at a beam entrance portion of the optical fiber unit and preventing the laser beams from focussing on one point in the optical fiber unit.
In preferred embodiments of the laser transmission system mentioned above, the pulse laser oscillator unit comprises a laser resonator including a rear mirror, an oscillator, an outgoing mirror and a pulse generator, the rear mirror having a beam reflection surface subjected to a matte finish treatment. The laser oscillator unit may comprise a laser resonator including a rear mirror, an oscillator, an outgoing mirror, a pulse generator, and a diffusion optical fiber through which an outgoing beam from the laser resonator passes. The laser oscillator unit may comprise a laser resonator including a rear mirror, an oscillator, an outgoing mirror, a pulse generator, and an optical fiber plate through which an outgoing beam from the laser resonator passes. The laser oscillator unit may comprise a laser resonator including a rear mirror, an oscillator, an outgoing mirror, a pulse generator, and a kaleidoscope through which an outgoing beam from the laser resonator passes. The laser oscillator unit may comprise a laser resonator including a rear mirror, an oscillator, an outgoing mirror, a pulse generator, and a beam transmitting plate for diffusion through which an outgoing beam from the laser resonator passes. The laser oscillator unit may comprise a laser resonator including a rear mirror, an oscillator, an outgoing mirror, a pulse generator, and a lens array through which an outgoing beam from the laser resonator passes.
The beam guide unit includes an optical fiber plate through which the pulse laser beam passes and a condensing optical device for condensing an outgoing beam from the optical fiber plate to the optical fiber unit.
The beam guide unit may include a kaleidoscope through which the pulse laser beam passes and a condensing optical device for condensing an outgoing beam from the kaleidoscope to the optical fiber unit. The beam guide unit may include a beam transmitting plate for diffusion through which the pulse laser beam passes and a condensing optical device for condensing an outgoing beam from the beam transmission plate to the optical fiber unit. The beam guide unit may include a lens array through which the pulse laser beam passes and a condensing optical device for condensing an outgoing beam from the lens array to the optical fiber unit.
The optical fiber unit has a central core having at least one end formed in a prismatic shape.
According to the invention, at least one of the pulse laser oscillator unit, the beam guide unit and the optical fiber unit is provided with means for reducing the coherence of a pulse laser beam, whereby a pulse laser beam of the high peak output can be transmitted while preventing the optical fiber from being damaged.
According to the invention according to the present invention of the aspect mentioned above, a pulse laser beam having the low spatial coherence and a wide spread angle indicative of directivity is radiated from a pulse laser oscillator unit. Due to this matter, such a pulse laser beam is not converged finely within the optical fiber even if the pulse laser beam is introduced into the transmission pulse laser beam. Thus, by employing this pulse laser oscillator unit as a beam source, a pulse laser beam having high peak output can be transmitted through the optical fiber without damaging the optical fiber.
Further, according to the invention according to the eighth to eleventh aspects, a pulse laser beam having high spatial coherence is introduced after being passed through a beam guide device to thereby reduce the spatial coherence thereof and to make the spatial intensity distribution thereof uniform. Owing to this matter, the pulse laser beam is not converged finely within the pulse laser beam and does not have a local intensity peak at the introduction (entrance) portion of the optical fiber, and the optical fiber is not damaged. Thus, by employing this optical fiber beam guide device, a pulse laser beam having a high peak output can be transmitted without damaging the optical fiber.
Furthermore, since the end portion of the core of a transmission optical fiber is formed into prism-shape, a pulse laser beam is guided to the optical fiber and is not converged finely within the optical fiber and the optical fiber is not damaged. Thus, if this optical fiber is used, a pulse laser beam having high peak output can be transmitted without damaging the optical fiber itself.
The nature and further characteristic features can be made more clear from the following descriptions made with reference to the accompanying drawings.